The present invention relates to a power supply or ballast for supplying a load at high frequency, particularly, but not exclusively, for high-intensity discharge (HID) lamps.
Ballasts with a frequency of the order of tens of kHz, typically approximately 20 kHz, are used for the supply of high-intensity lamps, for example metal halide lamps. One of the main problems encountered in the power supply of this type of lamp arises from the fact that resonance phenomena are triggered in the lamp at the power supply frequency, leading to a reduction in the lamp's life.
The resonant frequency varies from one lamp to another in a relatively wide range, and therefore it is impossible to design the ballast in such a way that the power supply frequency automatically excludes the resonant frequency of the lamp.
Various systems have been designed in an attempt to solve the problem mentioned above. In particular, it has been proposed that the lamp should be supplied at a variable frequency, instead of a constant frequency, to prevent the triggering of resonance phenomena. A mathematical analysis of this type of solution is set out in Laszlo Laskai et al., "White-noise modulation of high-frequency high-intensity discharge lamp ballast", 1/1994 IEEE, p. 1953 ff. In this article, to which reference should be made for the analytical examination of the mathematical aspects, it is proposed that the lamp should be supplied with a randomly variable frequency or phase. This requires a rather complex and expensive electronic circuit which is added to the PWM circuit which generates the switching signals.
The object of the present invention is to provide a circuit of a different design which is much simpler and less expensive, to solve the problem of resonance in HID lamps or in loads presenting similar problems supplied by high-frequency ballasts.